Wood pulps are generally produced through multistep processes. Initially, logs can be subjected to grinding in which the logs are forced against a rotating abrasive stone which separates the fibers from the log and also the wood cell matrix. In a refining process, wood chips are fed between two metal discs, with at least one disc rotating. In both cases, essentially all of the constituents of wood are retained in the pulp that is eventually produced. Such pulp contains fiber bundles, fiber fragments and whole fibers. A lack of uniformity of pulp and constituents and the presence of lignin in the pulp give it certain desirable qualities, such as yield, paper bulk and opacity as well as good printability. The pulp also has less desirable properties for some paper types, such as low strength, relatively coarse surface and a lack of durability.
Chips to be refined can be destructured and impregnated with chemicals or enzymes prior to further mechanical treatment. This can help increase pulp quality or reduce energy consumption. These methods create slightly different pulps and also vary with the species of wood, quality of the wood, processing conditions and the amount of energy applied. Various forms exist: thermomechanical pulping (TMP), refiner pulping, stone groundwood pulping, etc.
In TMP, steam is added to the chips being refined to facilitate pulping and lower electricity consumption. Steam is also produced during refining and heat recovery systems can help recoup some of the energy cost of the process. The electric motors used to operate these refiners require very large amounts of power. The TMP process generally involves several refining stages to produce a desirable pulp. However, only a small portion of the energy used in each refining stage is actually used to separate and develop the fibers. Screening is used after or between refining stages to separate adequately refined fibers from longer, coarser fibers. These tougher fibers are sent to “rejects” refiners for further development. Depending on the quality of refining, the amount of rejects needing additional refining can be and usually is significant.
Woody biomass used in these mechanical pulping processes contains cellulose, hemicelluloses, lignin and extractives in varying amounts throughout the ultrastructure of its fibers. These various components act in conjunction to give these substrates mechanical strength and resistance to degradation. By selectively removing or altering certain components, it is possible to reduce the amount of energy required to separate and refine these fibers. The patent literature describes various approaches using different enzyme mixtures. For example US Patent Publication No. 2005/0000666, of Taylor et al., describes the use of mannanase and xylanase. Certain treatments have been found to significantly impact paper strength properties which have limited their applications. U.S. Pat. No. 5,865,949, of Pere et al., describes a process using an enzyme mixture containing endo-β-glucanase (EG), a limited mannanase and cellobiohydrolase (CBH) activity which reduces the negative effects on paper strength. U.S. Pat. No. 6,099,688, of Pere et al., describes the use of isolated cellobiohydrolase to increase the amount of relative amorphousness of the cellulose within the fibers. This process is said to cause even less damage to paper properties.